KINEMATIC HISTORY OF DUCTILE AND BRITTLE DEFORMATION AT THE BLUE RIDGE-PIEDMONT BOUNDARY, CENTRAL VIRGINIA
In the Howardsville area, a ~5-km wide transition zone of steeply dipping and strongly deformed rocks separates the EBR and WP. Strain and kinematic indicators record strike-parallel (NE/SW) elongation, and general shear with an overall dextral transpressional geometry. Two Triassic half-grabens (Scottsville and Midway Mills basins) cut the Paleozoic structures, and are bounded to the west by NE-striking, SE-dipping normal faults. A suite of N- to NNW-striking Jurassic dikes intrudes older rocks and structures.
Rocks in both the EBR and WP are cut by a steeply dipping, WNW- to NW-striking joint set absent in the Mesozoic rocks. This major fracture set formed with the maximum principal stress oriented NW-SE, and likely developed during late Alleghanian (~280 to 300 Ma) regional shortening. The youngest brittle deformation features occur as sub-vertical joints in Mesozoic rocks, and include a NE-striking set cut by a NNW-striking set. The NNW-striking set is parallel to Jurassic diabase dikes, likely requiring a 30˚ to 40˚ rotation of the principal stress direction between the late Triassic and Jurassic.